The 34-year-old actress was spotted wearing an engagement ring during a grocery shopping trip in Los Angeles this week alongside Tom.
E! News reports that Tom popped the question back in May.
Lizzy and Tom met in London back in January 2015 and they made their first appearance together earlier this year.
While Lizzy is best known for her work in the movie Mean Girls and on the Showtime series Masters of Sex, Tom is the star of the series DaVinci’s Demons and he will soon be seen on Amazon’s The Collection.
Congratulations to the happy couple!